The IR optical transport system will be designed to handle beam powers of up to 5 kW to allow for future growth capability.


Full-power output should be directable to any one of the six user areas provided the appropriate interlock chains are engaged. In addition, low-power pickoffs will be provided to each or any area simultaneously with high power to a different area.


Power pickoff will be fail-safe with full power capability in an integral optical dump. Optical dump shall exhibit less than 10-4 backscatter into any occupied area. This is necessary for operational safety.


FEL optical diagnostic monitoring shall be provided consisting of a power monitor, a wavelength monitor, a pulse length monitor, and an image of the transverse beam profile at an appropriate pickoff point. Data from these will be provided for each user area and the FEL control room to optimize the FEL operation for particular tests.


Low-power pickoff will provide 10-3 power to all user areas simultaneously. A low-power (5 mW) HeNe beam collinear with anticipated FEL beam will also be provided.

Operating Pressure

Pressure is to be better than 10-4 torr. Quick vacuum reconnects are to be used in user areas. This is set by need to avoid absorption on atmospheric molecular lines.


An interlock is to be provided on each user lab door, with logic separately in each area. Redundant fail-safe shuttering control is to be provided. Master enable is to be provided by the FEL control room.


Outcoupling to room air is to be with a Brewster window of CaF2.

Beam Quality

Transport shall not degrade overall beam quality by more than 0.1 times diffraction limit. This requirement is easy, provided mirror absorption is not large and if the number of bounces is minimized (<6). Large optical trains can provide modest beam distortions if care is not taken. At the longest wavelengths diffraction will pose a problem in achieving this goal. Mirrors must be at least 5 optical waists in diameter.

Total Absorption

This is necessary to minimize spurious losses in the system and reduce what is available to users. In the IR region it should be easy to keep losses to <20% given the good reflectivities. An exception might be at the longer wavelengths where diffraction could cause added losses if care in the design is not taken.


Remote alignment will be provided to set up the transport and optimize it during laser operation. A range of 1 mrad in both directions is generally sufficient to deal with unexpected mounting movements.

Spurious Optical Modes

Scatter, etc., shall be held to better than 10-3 out of the user aperture in order to minimize safety issues and provided a reasonable degree of user control on process variables.

Mode Size, Configuration

The mode will be Gaussian with a radius ranging from 1.016 - 1.83 cm.

Rayleigh Range

The beam will be imaged at infinity. The Rayleigh range will be in excess of 150 m.